1. Field of Invention
The present invention relates to thermoplastic sheeting consisting of a pair of sheets or layers spaced apart and interconnected by longitudinally extending ribs so that the interior of the sheeting contains a plurality of longitudinally extending passageways. More particularly, it relates to thermoplastic sheeting having improved joint structures at the contacting points of the sheets and ribs which enhance the tear strength, flat crash resistance and cross elongation of the sheeting. The present invention also relates to the process for production thereof.
2. Description of the Prior Art
Hollow light weight panels made of thermoplastic resin which may be used to replace paper corrugated board are already known to those skilled in the art. A prior art method for the manufacture of hollow structure boards is to integrally mold a sheet with a plurality of ribs extending from the surface of the sheet and then laminate another sheet of thermoplastic resin to the tops of the ribs by bringing the sheet and the tops of the ribs together under heat-softened conditions such that they heat bond to one another. There is, however, an inherent drawback to the prior technique in that only a slight pressure may be applied when the constituents are united together by fusion state connection at their mutually contacting parts. As a result, the fusionally solidified joints of the constituent members represent naturally weaker points than other parts of the thus produced panel or board.
To overcome the problem of weak joints in the prior art technique, it is preferred to integrally extrude the two sheets and the plurality of ribs of the hollow plastic board through an extrusion orifice having a corresponding orifice configuration. This process was previously described in U.S. Pat. Nos. 3,274,315 and 3,792,951. Among the various types of hollow boards of thermoplastic resin as described in U.S. Pat. No. 3,274,315, the embodiment consisting of a pair of sheets or layers which are flat and substantially parallel, spaced apart and interconnected by longitudinally extending ribs, which are substantially vertical to the two flat sheets, is the most widely used, since this embodiment has good flat crush resistance and uses less thermoplastic material. However, undercuts regularly occur at the joint area of the two flat sheets and the longitudinally extending ribs of this configuration. Die lines are also often observed at the joint areas since the deposition of carbon easily occurs at the corner of the mandrels of the die. Both the undercuts and die lines deteriorate the tear strength, flat crush resistance and cross elongation of the hollow boards of the thermoplastic resin.
In order to overcome the foresaid shortcomings and to further improve the mechanical strength of the hollow thermoplastic resin board mentioned above, a renovation of the joints of the ribs and flat sheets of the board and its corresponding extrusion process are disclosed in this invention which is neither taught nor rendered obvious by the prior art.